As someone who gets dragged into disciplinary hearings a lot to investigate people in your position, a few things to check:
1. Company policies or procedures clearly state the route for requesting leave, having it authorised and recorded.
2. YOU have been given the opportunity to read and understand the policies/procedures in the past. If not they've messed up, regardless of what you did.
3. The company stick to the policy/procedure at all times, this includes all managers you deal with, if it's incoherent or simply ignored then again this is the company at fault, not you.
4. Check with co-workers that they've never been in your position, asked to write it in with not written confirmation given from management, if so then ask if they'll be a witness to prove management are not following procedure.
Bottom line is that without documentation neither side can really claim to be in the right here but at the same time they can't discipline you if the company has no uniform practice which is stuck to rigidly by management.
Lastly the one most people forget about:
5. The disciplinary process you're being taken through is within the company policies and is being followed to the letter, if they mess this up then you get a free get out of jail card.
Honest advice from being through these, if you did wrong - put your hands up and admit it at the first opportunity, if not stand your ground, check the points above and stick it to them. I've seen a lot of staff disciplined far more heavily simply because it was clear they broke the rules and they didn't admit it until the last opportunity to do so, wasting hundreds of hours of peoples time, but at the same time I've seen people get completely vindicated when the company was proven to be failing to follow it's own policies.